1. Field of the Invention
This invention is directed to an oil-soluble product useful in lubricating oil compositions. More particularly, this invention is directed to a star-shaped polymer having the properties of both a viscosity index-improver and a dispersant.
2. Description of the Prior Art
The newer engines place increased demands on the lubricants to be employed. In the past a number of different additives have been added to lubricating oils to improve such properties as viscosity index and dispersancy. One such additive added to lubricating oils to improve viscosity index is a two-block copolymer having the general configuration A-B where A is styrene and B is hydrogenated isoprene. See generally U.S. Pat. Nos. 3,763,044 and 3,772,196. A VI improver having greatly improved mechanical shear stability is the selectively hydrogenated star-shaped polymer disclosed in U.S. Pat. No. 4,116,917.
Significant reductions in cost can be made by employing a single additive that improves a number of lubricant properties. For example, in U.S. Pat. No. 4,141,847 a selectively hydrogenated star-shaped polymer is reacted first with an alpha-beta carboxylic acid, anhydride or ester, and then the product is reacted with an amine to form a dispersant-VI improver. Likewise, in U.S. Pat. No. 4,077,893 a similar product is obtained where an alkane polyol reactant is employed in place of the amine reactant to form a dispersant-VI improver. Still further, in the copending patent application by Rudolf J. Eckert, entitled "Lube Oil Additive," Ser. No. 203,073, filed Oct. 31, 1980, having a common assignee, a hydrogenated star-shaped polymer is reacted with a nitrogen containing polymerizable organic polar compound to form a dispersant-VI improver. The processes to form the above three products all have certain shortcomings. In each of the above described patents, the synthesis process involved an additional step whereby the star-shaped polymer is subjected to either free radical polymerization initiators, such as, tert-butyl hydroperoxide and tert-butyl benzoate or a high temperature condensation reaction between an .alpha.-.beta. unsaturated carboxylic acid or derivative and the residual olefin bonds in the star-polymer. The acidic derivatized site would then be reacted with an amine or alkane polyol. The high temperatures required for the free radical process (140.degree. C.) and condensation processes (180.degree.-250.degree. C.) add higher energy requirements for their manufacture and the additional reaction time as well as high temperatures increase the likelyhood of unwanted side-reactions such as cross-linking and chain-scission of the polymer. In each case the addition of a polar molecule, and more specifically a nitrogen-based molecule to the star-polymer backbone allows for the attainment of dispersant properties. Further process difficulties are encountered in controlling the degree of grafting and reproduceability of the functionalization reaction.
A new lube additive has been found that has significantly improved property advantages over the prior art additives.